This invention relates to an method and apparatus for wavelength-division multiplexing, particularly to extremely dense wavelength division multiplexing in a high capacity optical fiber communication system.
Wavelength division multiplexing (WDM) in an optical fiber communication system is a means for achieving increased information capacity by combining a number of channels in parallel in the same fiber. WDM provides high information capacities in a single fiber by combining a number of lower bit-rate channels. This reduces the requirement for high bit-rate transmitters, receivers and electronics.
The number of channels in a WDM system can be increased by reducing the channel spacing, or by increasing the system""s useable bandwidth. However, this can lead to increased transmission impairments that limit the application of WDM in communication systems. One fundamental limitation for very dense WDM transmission is optical nonlinearity. In particular, four photon mixing (FPM) nonlinearity, also known as xe2x80x9cfour wave mixing,xe2x80x9d can cause cross-talk between channels and excess attenuation or power loss. FPM appears as a fluctuating gain or loss due to constructive and destructive interference entailing signals of different channels. Thus, when more than one optical signal propagates in a fiber, FPM products are generated which can interfere with the information-bearing optical signals. These disadvantages associated with FPM have been described in xe2x80x9cfour-proton mixing and high-speed WDM systemsxe2x80x9d by R. W. Tkach, A. R. Chraplyvy, F. Forghieri, A. H. Gnauck, and R. M. Derosier, Journal of Light Wave Technology, May 1995, pp. 841-849.
FPM nonlinearity can be particularly severe in systems with equal channel spacing and in fiber with low dispersion such as dispersion-shifted fiber or TrueWave fiber near the low-dispersion end of the specifications. This problem has been observed in systems in which the short wavelength signals nearest the zero dispersion wavelength experience FPM degradation. FPM has also been observed where an attempt is made to increase bandwidth on existing fiber such that transmissions are made outside of the specified optimum bandwidth in which dispersion is lowest. Waveform distortions increase for those channels located far from the system""s zero dispersion wavelength. Waveform distortions also increase as the spacing between channels decreases.
Some efforts have been made to address the transmission impairments caused by FPM nonlinearity. The prior art discloses that orthogonal polarization launch can reduce the nonlinear channel interactions from FPM. This result was observed in a 320 Gb/s 64 channel transmission experiment using a 512 km amplifier chain in a circulating loop. Sixty four CW lasers were tuned on a 0.3 nm grid from 1540.6 nm to 1559.5 nm. The odd and even wavelengths were combined separately in a series of directional couplers. Each set of wavelengths first passed through an amplitude modulator, then through an amplitude and a phase modulator driven by a 5 GHz. sinusoidal clock. The transmitter generated 105 psec raised cosine pulses with the optical phase modulation adjusted to give maximum chirp aligned with the center of the pulse. The odd and even channels were then combined in a polarization dependent coupler.
One problem with this approach is that it is expensive to implement. It relies on the use of costly combiners and directional couplers to combine the orthogonally polarized channels. Another problem is that the combiners are physically large, making it difficult to implement them in existing WDM systems.
According to the invention there is provided an inexpensive and compact method of reducing the negative effects of four photon mixing in a wavelength division multiplex optical communications system comprising the steps of providing an arrayed waveguide router, and coupling channels which experience four photon mixing to said router using polarization-maintaining input fibers.
The invention also provides an inexpensive and compact apparatus for reducing the negative effects of four photon mixing in a wavelength division multiplex optical communications system, the apparatus comprising an arrayed waveguide router, and a plurality of input fibers, wherein one or more of said fibers are polarization-maintaining input fibers for coupling short wavelength channels to said router.